The invention relates to a dye-containing layer of a film-forming polymeric binder, in which the dye is dissolved or finely divided in the binder matrix.
In technology, in particular in the field of optical filters and optical information recording elements, there exists a strong need for very thin dye-containing layers which can be easily provided on a substrate and which have a high optical density with a small thickness.
Layers are known which consist entirely of dye and which can be provided on a substrate by means of a vapor deposition process in a small layer thickness of, for example, 0.5 um and with an optical density of approximately 0.4. Due to the vapor deposition process, the choice of the dye is restricted. For example, dyes having a low vapor pressure, for example ionic dyes, cannot be used. The same applies to dyes which are thermally unstable.
There are also known dye-polymer layers in which the dye is dissolved or finely divided in a polymeric binder. The layers can be obtained in various manners.
According to a conventional method, the dye is dissolved in liquid monomers. The solution is provided on the substrate to be coated in a thin layer and the monomers are then polymerized. For example, liquid monomers, for example acrylic acid esters, which can be polymerized with light, for example ultraviolet light, may be used. The resulting layers contain a comparatively small quantity of dye because only a small amount of dye, for example at most 5% by weight, can be dissolved in the liquid monomers. The result of the small concentration of dye is that comparatively thick layers in a thickness which is significantly more than 1 um are required to obtain an acceptable optical density, of for example, 0.4 and more.
Furthermore there are known dye-containing layers of a film-forming polymeric binder in which the dye is dissolved or finely divided in the binder matrix. These layers are manufactured by dissolving both the polymeric binder and the dye in a suitable solvent or mixture of solvents, then coating a substrate with a thin layer of the solution and finally evaporating the solvent in air or in a vacuum. By variation of the solvent, a large variety of dyes may be used. The quality of the layer or film is much less dependent on the concentration of the dye than in the above-described layers obtained by using liquid monomers.
However, it has not been possible to manufacture dye-containing layers of a film-forming polymeric binder in which a large quantity of dye is dissolved. The known layers must therefore be used in comparatively large thicknesses of more than 1 um so as to be able to realize a sufficiently high optical density of the layer.
The layer thicknesses mentioned in literature vary from a minimum value of 1-2 um (Science 154, pp. 1550-1966); W. C. Meyers, Proc., 3rd Int. Cong. Reprography London, Conf. paper 39 (1971); J. appl. Photo. Eng., 3, p. 40 (1977), to a value of 15-50 um German Off. 19 21 245; c.a. 82, 37 59t (1975); C.A. 82, 49 883x (1975); J. Appl. Chem. Biotechnol. 22, p. 799 (1972).